Antennas designed for operation in cellular radiotelephone systems and personal communication systems (PCS) are often configured to serve only a single sector of a cell. Such antennas are often implemented as panel antennas because control of horizontal beamwidth is more easily implemented in a panel configuration, thus making the panel antenna nearly ideal for sectorized cell sites.
Because cell sites in both cellular and PCS applications serve primarily mobile and portable subscribers, relative motion between the cell site and the subscriber units must be taken into account. In urban environments, there may also be regions within each cell site where coverage is difficult because of tall buildings or other obstructions. Such obstructions can give rise to multipath transmissions and result in interference.
One way to address such coverage difficulties is to provide diversity reception. Diversity refers to the concept of providing more than one antenna for communication between the cell site and subscribers. Usually, diversity is provided for the receive path only, although transmit diversity can also be useful in addressing certain propagation problems. Diversity is also generally provided at the cell site only, but many mobile subscribers implement diversity reception via the provision of multiple vehicle antennas.
Polarization diversity can be brought into play to address communication difficulties. Polarization refers to the orientation of the electric field (E-field) of a transverse electromagnetic (TEM) wave with respect to the earth's surface. A half-wave dipole oriented perpendicular to the earth's surface will exhibit vertical polarization, and the same dipole antenna rotated so that it is parallel to the surface of the earth is said to be horizontally polarized.
Antenna systems that include vertically and horizontally polarized arrays are known for cellular and PCS communication systems. Often, the vertically polarized array is used for transmitting, with diversity switching provided between the vertical and horizontal arrays to provide polarization diversity for the receive path. However, this configuration does not provide optimum coverage for some types of communication units.
Subscribers using portable communication units are usually careless regarding the orientation of the antenna during a conversation. Those who are used to communicating over portable two-way radios, such as police, fire protection personnel, etc., will often consciously attempt to maintain a vertical antenna orientation during radio communication. This is not true for members of the general public, however, who use portable cellular and PCS units just as though they were landline telephone sets, and are often unaware that radio communication is occurring as a consequence of the telephone calls they place and receive with their cellular and PCS "phones."
These users often have the antenna oriented at a 45 degree angle with respect to the earth's surface. This is something of a "natural" angle considering the design of modern portable units. With the unit held to the ear in normal conversation position, the antenna tends to be oriented at about 45 degrees with respect to the horizontal.
Polarization diversity with antennas oriented at plus and minus 45 degrees are very effective for communicating with a subscriber population consisting largely of portable units. Conventional 45 degree polarization antennas comprise two dipole element arrays one at plus 45 degrees and one at minus 45 degrees from the earth's surface. The dipole element arrays are arranged as dipole pairs, with each dipole pair consisting of a plus 45 degree dipole element from one array and a minus 45 degree dipole element from the other array which intersect at their midpoints.
These types of antenna provide suitable 45 degree polarization, however the radiation pattern beamwidth is poor, typically around 90.degree.. The beamwidth size effects the gain of the antenna. A radiation pattern having a smaller beamwidth is projected farther forward along the horizontal plane and less along the vertical plane. Thus, the smaller the beamwidth, the larger the antenna gain.
Prior attempts to reduce the beamwidth size of an antenna system typically compromise the isolation between antenna elements in order to decrease beamwidth size. However, proper isolation is an important element of polarization diversity antenna systems. Accordingly, a need arises for a 45 degree polarization diversity antenna system which has a reduced radiation pattern beamwidth and which does not compromise the isolation between the dipole elements comprising the antenna system.